In the constant pursuit of improved safety for automobiles, for example, manufacturers have installed telescoping components which cooperate with the steering column upon frontal impact of the automobile to lessen the severity of injury to the driver. These mechanisms fulfill an important objective but add to the total cost and weight of the vehicle, the latter (weight) being a factor in the amount of energy required to move the vehicle.
In addition to telescoping components, flexible shafts have been used in steering mechanisms to also lessen driver injuries. Flexible shafts improve the ability of the steering column to absorb energy in a crash situation. The flexible shaft has one of its ends operably connected to the bottom of the steering shaft while its other end may be connected to some suitable tuned coupling means capable of absorbing considerable road induced shake. Such flexible shafts, over a period of years of being constantly torqued in both directions, have catastrophically failed resulting in a complete loss of steering control. Since driving habits of individual drivers vary widely as well as conditions under which the automobiles are driven, life expectancy of the flexible shafts used therein is not always predictable. Such flexible shafts may suddenly fail on a rapid turn of the steering wheel.
The present invention provides visual means for indicating the condition of the flexible shaft. Thus, the red or danger portion of the indicating means may be caused to light when the flexible shaft has sufficiently deteriorated, or its condition may be observed by electronic means when desired.
The indicating means is electrically connected to a piezoelectric film material secured to the outermost layer of windings of the rotatable flexible shaft. The film material is preferably polyvinylidene fluoride, referred to hereinafter as PVDF, suitably Kynar, a trademark product of Pennwalt Corporation, Philadelphia, Pa., assignee of the present invention. Stress applied to the piezoelectric film generates millivoltages thereacross when the flexible shaft is torqued in either direction in response to turning of the steering wheel. Of course, a flexible shaft approaching failure will exert more stress on the film, and hence, higher millivoltages will be generated than from torquing a flexible shaft in relatively good condition. The millivoltages are conventionally amplified and the amplified signals directed to the indicator for indicating safeness of the flexible shaft.